While carrying the amulet, casting spells takes more power than usual. The penalty is between 1 and 2n power points, where n is the normal power cost of the spell cast. This means spell costs will be doubled on average, and may use up to three times as much power in the worst case.[12]

If you try to go upstairs in Gehennom while carrying the Amulet of Yendor, the mysterious force may instead teleport you down 0 to 3 levels (a level change of zero teleports you somewhere else in the level). The chance depends on your alignment.

If you offer the amulet on the correct high altar on the Astral Plane, you ascend and win the game. Offering the amulet on another god's high altar will also end the game: your current god is angered, but the altar's god allows you to escape in celestial disgrace. This is effectively an ascension, but is less prestigious because it removes the need to find the correct altar.

Attempting to offer the amulet on any other altar gives the message “You feel an urge to return to the surface.”, or if hallucinating, “You feel homesick.” There is no penalty for this, even if you attempt to do it at a cross-aligned or unaligned altar.

You cast a spell while carrying the amulet. Between 1 and 2n extra Power points were used, where n equals to the Power originally needed to cast the spell. This Power drain is incurred before the normal Power used for the spell is calculated, resulting in spells occasionally being unable to be cast due to a lack of energy. Ideally, even with 0% spell failure, you will require 3n Power to ensure a spell always works.

It is possible to find a Cheap plastic imitation of the Amulet of Yendor at several places in the Mazes of Menace. These generally appear to the player as "The Amulet of Yendor," but messing up and offering one of them to your god carries a penalty to luck, and possibly alignment and your god's anger (if you attempt to offer a known fake). But the real danger lies in becoming confused by the abundance of fakes and carrying one off instead of the real amulet if, for instance, the Wizard steals it from you. There are several ways to avoid this fate. Many players will immediately #[name] the real Amulet "REAL" or something similar immediately upon claiming it from the High Priest. Another technique is to attempt to put the Amulet into a container; the real Amulet will resist, but a fake will go in.

This mysterious talisman is the object of your quest. It is
said to possess powers which mere mortals can scarcely
comprehend, let alone utilize. The gods will grant the gift of
immortality to the adventurer who can deliver it from the
depths of Moloch's Sanctum and offer it on the appropriate high
altar on the Astral Plane.